firmware: tegra: bpmp: Do not support big-endian

The CPU and BPMP inter-processor communication code is only partially
endian-aware, so it doesn't work properly when run on a big-endian CPU
anyway. Running Tegra SoCs in big-endian mode has also never been
supported, especially not on those with 64-bit ARM processors.

If big-endian support ever becomes necessary this can be added back but
will need additional fixes for completeness.

Signed-off-by: Thierry Reding <treding@nvidia.com>